This invention refers to a device in flexographic machines, for making an adjustment between an inner shaft and an extractable jacket having an inner diameter larger than the outer diameter of the shaft, with the double purpose of supporting the jacket along the entire length of the shaft, and of increasing the stiffness of the jacket.
In the modern flexographic machines are often used extractable jackets on which is sticked or vulcanized the printing plate. Thanks to these jackets, a change of the subject to be printed may be effected in a few minutes. In order to ensure a perfect adhesion between the jacket and the supporting shaft (the so called mandrel), the jackets are manufactured with an inner diameter slightly smaller than the outer diameter of the mandrel, in order to provide an assemblage with interference. The operations for inserting and extracting the jacket are made easier by blowing compressed air between the mandrel and the jacket, whereby the jacket dilates and an air cushion is provided between both parts.
When, as it often happens in the printing machines for flexible packings, it is needed that the print size (and therefore the outer diameter of the jacket) is modified, jackets having different thickness are used. However, the only modification of the jacket thickness allows covering only a limited field of print size modifications. When a larger print size modification is required, use is made of a jacket having a larger diameter, and an adapter is introduced between the mandrel and the jacket. The presently known adapters may be of two different types: (1) adapters mounted with interference by means of an air cushion; and (2) adapters blocked at their end portions by means of hydraulic expansion of the shaft.
An adapter of the type (1) should be manufactured with a relatively elastic material (usually a plastic matter) in order to allow its dilation under action of the pneumatic pressure. This kind of adapter has the advantage of ensuring a close contact along the entire length, both between the mandrel and the adapter and between the adapter and the jacket, but, being elastic, it has the drawback that it cannot contribute to the stiffening of the mandrel, as it would be highly desirable.
An adapter of the type (2) is manufactured with a stiff material (for example, steel and carbon fiber) and it is rigidly connected to the mandrel at its end portions, whereby it contributes to the stiffening of the mandrel. However this type of adapter has the drawback that, in its central region, it does not rest against the mandrel, whereby the jacket carrying the print plate is not well supported in its central region and therefore, under the print pressure, it can give rise to troublesome flexions and vibrations.